Gastric cancer causes more deaths worldwide than any other cancer, with the exception of lung cancer. Though the incidence of gastric cancer in Western nations is relatively low, patients in this region rarely present at an early stage of the disease. Consequent high mortality rates create a pressing need for improved treatment. Molecularly targeted therapies such as kinase inhibitors are under intensive investigation for treatment of gastric cancer. Of particular interest are drugs inhibiting HER2 and EGFR kinases, receptor tyrosine kinases of the human epidermal growth factor receptor (HER) family that demonstrate aberrant activity in subsets of gastric cancer. Currently, the FDA has approved the use of the HER2 inhibitor trastuzumab for the treatment of some gastric cancers, and several EGFR inhibitors are being evaluated in phase II and III clinical trials for efficacy in gastric caner therapy in combination with cytotoxic chemotherapy. With the advent of these targeted agents has arisen an urgent need to accurately determine patient eligibility for anti-HER treatment. This work proposes to develop a tool that addresses many of the limitations of current diagnostic technologies by directly assaying aberrant activity of HER2 and EGFR kinases at the single cell level. Highly selective peptide substrates of HER2 and EGFR kinase will be generated via iterative combinatorial optimization of a peptide scaffold using libraries of non-native side chain functionalities. Single cells loaded with fluorescently labeled substrates will be lysed and analyzed with a single-cell analysis system comprised of a fluorescence microscope and an exquisitely sensitive capillary electrophoresis system with laser-induced fluorescence detection. Ideally suited to small clinical biopsies, this technology will be applied both to gastric cancer models and to patient samples in order to quantitatively evaluate EGFR and HER2 signaling dysregulation at the single-cell level.